1. Field of the Invention
The present invention relates to control circuits for a kidney dialysis machine and more particularly to a backup control circuit connected to a main circuit for a modular proportioning kidney dialysis machine.
2. Description of the Prior Art
Heretofore there has been utilized in a modular proportioning kidney dialysis machine, a main control circuit which has inputs connected to various sensors which sense aberrant conditions in the dialysate and aberrant conditions in the arterial and venous blood pressure as well as an excess of blood leakage into the dialysate. More specifically, one sensor senses when the conductivity, temperature, flow and/or negative pressure of the dialysate is out of a predetermined range. When this occurs an error condition or alarm signal is generated which is utilized by the main control circuit to effect closing of a dialysate operate valve and opening of a dialysate bypass valve to bypass the dialysate around the kidney dialysis machine.
Another sensor senses when the arterial blood pressure and/or the venous blood pressure of the patient are outside of predetermined pressure ranges. When an aberrant or out of range condition occurs, an error or alarm signal is generated which is sensed by the main control circuit and utilized to stop operation of the blood pump.
Further, the waste dialysate from the kidney dialysis machine is passed by an optical sensor which senses the optical density of the dialysate to determine the amount of red blood cells leaking into the dialysate across a membrane in the kidney dialysis machine. When the amount of blood leakage exceeds a certain value indicating an aberrant operating condition, another error or alarm signal is generated which is sensed by the main control circuit to cause stopping of the blood pump.
The main control circuit also includes an override switch so that an operator, after noting the occurrence of an aberrant operating condition, can turn the blood pump back on to pump the patient's blood back into the patient. Also there is provided a purge switch, a clean switch, an interlock switch associated with the clean switch and a sterilize input for sensing a sterilize signal indicating that the kidney dialysis machine is to be purged and cleaned.
A main control circuit functioning in the manner described above has a number of components, which because of the number of components and because of the cost of components, are normal reliability components and not extra high reliability components. As a result, and although the failure rate of these components is very low, it is possible that a component failure could occur which would result in improper or malfunctioning of the main control circuit. More specifically, the main control circuit, because of a component failure, could conceivably fail to close the operate valve, open the bypass valve and/or stop the blood pump.
To ensure against such a possible failure of the main control circuit, the backup control circuit of the present invention utilizes extra high reliability components and is connected to the main control circuit in such a way as to take over operation of the control circuit in case there is a component failure in the main control circuit thereby to ensure proper closing of the operate valve, opening of the bypass valve, and stopping of the blood pump when an aberrant operating condition of the dialysate or blood is sensed.
The particular manner in which the backup control circuit functions to take over the operation of the main control circuit if the main control circuit does not operate within a predetermined time period after an aberrant condition is sensed, is more fully described hereinafter.